I didn't recognize the encoding as base-64. I actually looked at the encoding and spent a few seconds thinking about it. At that point I noticed that it used letters of both case and digits and so I tried to think of encodings that used such. base-64 and uuencode both do that and a quick test showed that base64 worked.

I didn't let some initial guess ("UTF-specific shenanigans") prevent me from trying to solve the problem. (: Just FYI.